Acidic and Electrosurface Properties of Binary tio2–sio2 Xerogels Using Epr of Ph‐Sensitive Nitroxides

The binary xerogels TiO2–SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2–SiO2 xerogels applica-tions. This research investigates acid–base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2–SiO2 xerogels, by the EPR of stable pH‐sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge asso-ciated with surface electrical potential (SEP) was equal to –196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. Тhe local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Funding: D.O.A., E.G.K., D.P.T. acknowledge the financial support from the Russian Foundation for Basic Research (RFBR) grant 18‐29‐12129mk, I.A.K. acknowledges support from the Ministry of Sci‐ ence and Education of the Russian Federation grant No. 14.W03.31.0034 for the nitroxide synthesis

DIRECTED DESIGN OF NOVEL ADSORBENTS BASED ON HALLOYSITE MINERA

The research work was supported by the Russian Foundation for Basic Research (Grant 18-29-12129мк)

Grafting of (3-chloropropyl)-trimethoxy silane on halloysite nanotubes surface

Modified halloysite nanotubes (HNTs-Cl) were synthesized by a coupling reaction with (3-chloropropyl) trimethoxysilane (CPTMS). The incorporation of chloro-silane onto HNTs surface creates HNTs-Cl, which has great chemical activity and is considered a good candidate as an active site that reacts with other active molecules in order to create new materials with great applications in chemical engineering and nanotechnology. The value of this work lies in the fact that improving the degree of grafting of chloro-silane onto the HNT’s surface has been accomplished by incorporation of HNTs with CPTMS under different experimental conditions. Many parameters, such as the dispersing media, the molar ratio of HNTs/CPTMS/H2O, refluxing time, and the type of catalyst were studied. The greatest degree of grafting was accomplished by using toluene as a medium for the grafting process, with a molar ratio of HNTs/CPTMS/H2O of 1:1:3, and a refluxing time of 4 h. The addition of 7.169 mmol of triethylamine (Et3N) and 25.97 mmol of ammonium hydroxide (NH4OH) led to an increase in the degree of grafting of CPTMS onto the HNT’s surface. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.This research was funded by Program 211 of the Government of the Russian Federation No. 02.A03.21.0006, RFBR grants 17-03-00641 and 18-29-12129 mk, the State Task from the Ministry of the Education and Science of the Russian Federation No. 4.9514.2017/8.9

Various parameters that effect on silanization of halloysite nanotubes by using (3-Glycidyl oxypropyl) trimethoxy silane

The work was supported by RFBR grants 18-29-12129mk

Electrostatic properties of inner nanopore surfaces of anodic aluminum oxide membranes upon high temperature annealing revealed by EPR of pH-sensitive spin probes and labels

Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria. © 2020 Elsevier B.V.Russian Foundation for Basic Research. Government Council on Grants, Russian Federation. U.S. Department of Energy. Ministry of Science and Higher Education of the Russian Federation. National Science Foundation. North Carolina State Universit

CATALYTIC BREAKDOWN OF NATURAL POLYSACCHARIDES BY ENZYMES IMMOBILIZED ON GAMMA ALUMINA

Meso- and nanoporous materials including nanoporous anodic aluminum oxide (AAO) membranes, mesoporous and nano- alumina, have been actively studied in various fields including heterogeneous catalysis and adsorption processes. In the previous works, we studied electosurface properties of these materials. Now we present the catalytic data on breakdown of such natural polysaccharides as xylan and chitosan specific enzymes immobilized on gamma alumina powders.Работа выполнена при частичной финансовой поддержке грантов РФФИ № 14-03-00898, 17-03-00641 и Минобразования РФ в рамках базовой части государственного задания, проект №4.9514.2017/8.9

OPTIMIZATION OF SOME RELEVANT FACTORS FOR GRAFTING ORGANOSILANES ON HALLOYSITE NANOTUBES

The work was financially supported by RFBR grant 18-29-12129mk and the State Task from the Ministry of the Education and Science of the Russian Federation nos. 4.9514.2017/8.9

Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson-Boltzmann Double Layer Theory

Chemical and physical processes occurring within the nanochannels of mesoporous materials are known to be determined by both the chemical nature of the solution inside the pores/channels as well as the channel surface properties, including surface electrostatic potential. Such properties are important for numerous practical applications such as heterogeneous catalysis and chemical adsorption including chromatography. However, for solute molecules diffusing inside the pores, the surface potential is expected to be effectively screened by counter ions for the distances exceeding the Debye length. Here, we employed electron paramagnetic resonance spectroscopy of ionizable nitroxide spin probes to experimentally examine the conditions for the efficient electrostatic surface potential screening inside the nanochannels of chemically similar silica-based mesoporous molecular sieves (MMS) filled with water at ambient conditions and a moderate ionic strength of 0.1 M. Three silica MMS having average channel diameters of D = 2.3, 3.2, and 8.1 nm (C 12 MCM-41, C 16 MCM-14, and SBA-15, respectively) were chosen to investigate effects of the channel diameter at the nanoscale. The results are compared with the classical Poisson-Boltzmann (PB) double layer theory developed for diluted electrolytes and applied to a cylindrical capillary of infinite extent. While the surface electrostatic potential was effectively screened by the counter ions inside the largest channels of 8.1 nm in diameter (SBA-15), the effect of the surface electrostatic potential on local effective pH was significant for the 3.2 nm channels (C 16 MCM-14). The smaller channels of C 12 MCM-41 (2.3 nm in diameter) provided the most critical test for the PB equation that is based on a continuum electrostatic model and demonstrated its inapplicability likely due to the discrete nature of molecular systems at the nanoscale and nanoconfinement effects, leading to larger spatial heterogeneity. © 2018 American Chemical Society.*E-mail: [email protected] (E.G.K.). *E-mail: [email protected] (A.I.S.). ORCID Alex I. Smirnov: 0000-0002-0037-2555 Author Contributions The manuscript was written through contributions from all the authors. All authors have given approval to the final version of the manuscript. Funding E.G.K., L.S.M., D.O.A., D.P.T., and A.N.T. acknowledge the financial support of the Program 211 of the Government of the Russian Federation no. 02.A03.21.0006, RFBR grants 17-03-00641 and 18-29-12129mk, and the State Task from the Ministry of the Education and Science of the Russian Federation nos. 4.9514.2017/8.9 and 4.7772.2017/8.9. A.M. and A.I.S. acknowledge the financial support of the U.S. DOE Contract DE-FG02-02ER15354 (modeling of electrostatic phenomena in the nanochannels and the final preparation of the manuscript). Notes The authors declare no competing financial interest